1. Field of the Invention
The present invention relates to a thermal recording material that forms color images thereon through reaction of a diazo compound and a coupler compound therein.
2. Description of the Related Art
Diazonium salt compounds have an extremely high chemical activity, and they react readily with phenol derivatives and active methylene-having compounds that are generally referred to as coupling components to form azo dyes. In addition, being sensitive to light, they decompose when exposed to light, and lose their activity. Accordingly, diazonium salt compounds have been used for many years for optical recording materials, for example, typically for diazo copies (see Principles of Photographic Science and Engineeringxe2x80x94Non-Silver Salt Photography, edited by the Photographic Society of Japan, published by Corona Publishing Co., LTD., 1982, pp. 89-117, pp. 182-201).
These days, in addition, diazonium salt compounds are applied also to recording materials that require image fixation, as they have the property of decomposing through exposure to light to lose their activity. One typical example is a photo-fixing, thermal recording material in which the recording layer contains a diazonium salt compound and a coupling component that are reacted under heat in accordance with an image signal applied thereto to thereby form the intended image and the image is then fixed through exposure to light (Koji Sato et al., the Journal of the Imaging Electronics Society of Japan, Vol. 11, No. 4, 1982, pp. 290-296).
However, in the recording materials of this type that contain a diazonium salt compound serving as a color-forming component therein, the chemical activity of the diazonium salt compound is extremely high, and the diazonium salt compound therein gradually decomposes under heat even in the dark to lose its activity. Therefore, the drawback of the recording materials is that their shelf life is short. In addition, the diazonium salt compound still remaining in the non-image background area of the recording materials decomposes to form a color stain while the materials are exposed to light for image fixation thereon, and, as a result, the non-image background area of the thus processed materials is stained. This is another drawback of the recording materials. Moreover, even in the finished materials in which the image formed has been finally fixed, the non-image area is poorly resistant to light, and therefore the stain in the non-image area often increases when the finished materials are left in sunlight or under fluorescent lamps for a long period of time. This is still another drawback of the recording materials.
Various methods have heretofore been proposed for improving the stability of such diazonium salt compounds. One of the most effective methods is to encapsulate a diazonium salt compound into microcapsules. Encapsulated in microcapsules, the diazonium salt compound is isolated from water and bases that promote the decomposition of the compound. In that condition, therefore, the diazonium salt compound is almost completely prevented from being decomposed, and the shelf life of recording materials that contain the diazonium salt compound in microcapsules is significantly prolonged (Tomomasa Usami et al., The Journal of the Electrophotography Society of Japan, Vol. 26, No. 2, 1987, pp. 115-125).
One general method for encapsulating a diazonium salt compound into microcapsules is as follows: A diazonium salt compound is dissolved in a hydrophobic solvent (to form an oily phase), and this is added to an aqueous solution of a water-soluble polymer (aqueous phase), and emulsified and dispersed by the use of a homogenizer or the like. In the process, a monomer or a prepolymer to form a wall of microcapsules is added to either one or both of the oily phase and/or the aqueous phase so that it is polymerized in the interface between the oily phase and the aqueous phase to form a polymer wall around the emulsified particles of the diazonium salt compound to thereby encapsulate the diazonium salt compound into the thus-formed microcapsules. The details of the method are described, for example, in Tomoji Kondo""s Microcapsules (by Nikkan Kogyo Shinbun, 1970) and Tamotsu Kondo et al""s Microcapsules (by Sankyo Publishing, 1977).
For the microcapsule walls to be formed, various compounds are usable, for example, crosslinked gelatin, alginates, celluloses, urea resins, urethane resins, melamine resins, and nylon resins.
In cases where microcapsule walls are made of urea resin or urethane resin that undergoes phase transition at its glass transition point and where the glass transition point of the microcapsule walls is higher than room temperature to some extent, the microcapsule walls are impervious to substances at room temperature but are pervious thereto at temperatures not lower than their glass transition point. Therefore, the microcapsules of this type are referred to as thermo-responsive microcapsules, and these are useful in thermal recording materials.
Specifically, a thermal recording material having, on a support, a thermal recording layer that contains thermo-responsive microcapsules with a diazonium salt compound therein and a coupling component as the essential color-forming ingredients ensures long-term stability of the diazonium compound therein. When exposed to heat, it readily forms a color image thereon, and when exposed to light, the color image formed is fixed on it.
As in the above, the technique of encapsulating a diazonium salt compound into microcapsules makes it possible to significantly improve the stability of thermal recording materials that contain microcapsules of a diazonium salt compound therein.
However, even in microcapsules, diazonium salt compounds could not be well stabilized because of the property intrinsic thereto, and no one has heretofore succeeded in attaining satisfactory long-term stability of thermal recording materials containing diazonium salt compounds. This is because diazonium salt compounds undergo optical decomposition when exposed to light, and the diazo group therein therefore decomposes in that condition to form photo-decomposition stains. In addition, color stains also increase, and, as a result, the whiteness in the non-image area of the photo-fixed materials lowers and the contrast between the non-color area and the color area thereof lowers.
Further, it is known that the reaction is not uniform and therefore gives various decomposed products depending on the ambient environment. Of tens or more different types of the decomposed products, those referred to as photo-decomposition stains absorb visible rays. If the stains of this type are significant, the whiteness in the non-recorded area of the photo-fixed materials lowers, and the contrast between the image area and the non-image area lowers. If so, the commercial value of the recording materials is greatly lowered. However, the mechanism of photo-decomposition of diazonium salt compounds is complicated, and the decomposed products from them are difficult to specifically identify. For these reasons, it has heretofore been said in the art that photo-decomposition stains from diazonium salt compounds are difficult to control.
Given that situation, various studies are made these days in the art for solving the problem of photo-decomposition stains in thermal recording materials so as to improve the long-term stability of the materials. For example, in Japanese Patent application Laid-Open (JP-A) No.8-324129, proposed is a photo-fixing, thermal recording material for which are used microcapsules of a photo-fixing diazonium salt compound along with a specific hydrophobic oil. It is claimed that the recording material proposed has good storage stability and, after photo-fixing, the whiteness in the background area of the material is high. It is further claimed that, even if exposed to light for a long period of time after image formation thereon, the whiteness in the background area of the material and the image stability thereof are still good.
In JP-A No. 11-078232, proposed is a thermal recording material that comprises a novel diazonium salt compound. This is for improving the stability of the diazonium salt compound itself used therein. Concretely, the diazonium salt compound used therein is so modified that its maximum absorption wavelength is shifted shorter than around 350 nm, and therefore the diazonium salt compound is stable in a long wavelength range longer than around 350 nm. The thermal recording material proposed contains microcapsules of the diazonium salt compound of that type, and its advantages are that the whiteness in the non-image area of the material exposed to light longer than around 350 nm such as typically a fluorescent lamp for image formation thereon is high, and the discoloration in the image area thereof is retarded.
However, depending on the condition in which it is stored, the fixless thermal recording material is still unsatisfactory in point of its raw stock storability and in point of its ability to prevent stains in the background area thereof after processed for image formation thereon, and in addition, the image storability of the processed material is also still unsatisfactory.
In JP-A No. 2001-138639, proposed is a thermal recording material that comprises a fixless diazonium salt compound. It is claimed that the ability of the material to form color images is good, and the lightfastness of the image-formed material is also good. However, even the recording material that contains such a fixless diazonium salt compound is still unsatisfactory in point of its image storability after being exposed to light, and there still remains room for further improvement in the recording material proposed.
The present invention has been made in consideration of the above-mentioned problems, and its object is to provide a thermal recording material of good image storability.
The object of the present invention as above is attained as follows.
In its first aspect, the present invention provides a thermal recording material including: a support; and, a thermal recording layer provided on the support, and including a compound represented by the following general formula (1), a compound represented by the following general formula (2), and a compound represented by the general formula (3): 
In general formula (1): R1, R2, R3 and R4 each independently represent a group selected from the group consisting of a hydrogen atom, halogen atoms, alkyl groups, aryl groups, xe2x80x94OR51, xe2x80x94SR51, xe2x80x94COOR51, xe2x80x94CONR51R52, xe2x80x94SO2R51, xe2x80x94SO2NR51R52, xe2x80x94COR51, xe2x80x94NR51R52, nitro groups and cyano groups; R51 and R52 each independently represent a group selected from the group consisting of a hydrogen atom, alkyl groups, aryl groups and acyl groups; R5 represents a group selected from the group consisting of a hydrogen atom, alkyl groups, aryl groups, xe2x80x94COOR53, xe2x80x94CONR53R54, xe2x80x94SO2R53, xe2x80x94SO2NR53R54, and xe2x80x94COR53; and R53 and R54 each independently represent a group selected from the group consisting of a hydrogen atom, alkyl groups, aryl groups and acyl groups. 
In general formula (2): R6 represents an alkyl group or an aryl group; one of X and Y represents Cxe2x80x94R7 and the other of X and Y represents N; and R7 represents an alkyl group or an aryl group.
Rnxe2x80x94Mmxe2x80x83xe2x80x83General Formula (3)
In general formula (3): R represents an anion with a valency from 1 to 3; M represents a metal ion with a valency from 1 to 3; and n and m each independently represents an integer from 1 to 3.
A second aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the compound represented by general formula (1) includes a solid content coating amount thereof from 0.1 to 0.8 mmol/m2.
A third aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the compound represented by general formula (2) includes a solid content coating amount thereof from 0.3 to 2.4 mmol/m2.
A fourth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the compound represented by general formula (3) includes a solid content coating amount thereof from 0.3 to 2.4 mmol/m2.
A fifth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the compound represented by general formula (2) includes a proportional amount thereof from 0.1 to 20 mol relative to one mole of the compound represented by general formula (1).
A sixth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the compound represented by general formula (3) includes a proportional amount thereof from 0.01 to 10 mol relative to one mole of the compound represented by general formula (2).
A seventh aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the anion with a valency from 1 to 3 represented by R in general formula (3) includes an anion selected from the group consisting of phenol anions, alcohol anions, mercaptan ions, carboxylate ions, sulfonate ions, thiocarboxylate ions, dithiocarboxylate ions, dithiocarbamate ions, thiocarbonate ions, phosphate ions, monoalkylphosphate ions, dialkylphosphate ions, phosphonate ions acetylacetone ions and salicylate ions.
An eighth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which M in formula (3) is selected from a group consisting of Zn2+, Fe2+, Fe3+, Ni2+ and Al3+.
A ninth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the compound represented by general formula (1) includes the following diazo compound (C). 
A tenth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the compound represented by general formula (2) includes at least one compound selected from the group consisting of the following coupler compound (F), the following coupler compound (G) and the following coupler compound (H). 
An eleventh aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the compound represented by general formula (3) includes at least one of the following zinc compounds (A) and (B). 
A twelfth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which at least one of the compounds represented by general formulae (1) to (3) is encapsulated into microcapsules.
A thirteenth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the compound represented by the general formula (1) is encapsulated in microcapsules.
A fourteenth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the thermal recording layer further includes an organic base.
A fifteenth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the thermal recording layer further includes a color-formation promoter.
A sixteenth aspect of the present invention is a thermal recording material subsidiary to the first aspect thereof, in which the thermal recording layer further includes a free radical generator.